Soil and Water in the Life of a Street Tree

This is the second post in the series Understanding the Urban Forest from the Ground Up.  Part one can be read here.

Unlike other forms of infrastructure, urban forests are also an ecosystem.  As a managed system, it grows and evolves over time, but has basic requirements that must be met in order to function as planned. It is a bit like an infantry unit; it will suffer casualties and face attrition over time.  Replacement candidates have to be cultivated and carefully selected to address specific weaknesses in the unit as a whole and must be able to adapt to changing circumstances and impending threats.  But regardless of tree selection, it all starts with the soil.  It’s easy to look down on the dirt and forget that it also is a living system that itself needs to be protected and cultivated.  We can plant a million trees, but unless we understand what is happening below the surface it will largely be a wasted effort.

 

Where is the Soil

Virtual trees can be stuffed into the ROW cross section, real ones cannot.

One of the leading problems facing urban trees is the lack of space.  Specifically, a lack of porous, moist, and oxygenated soil for roots to inhabit.  Let’s look at one of the most common planting situations in American cities today, the boulevard. Space is always at a premium in the urban right of way, and there is a lot of competition within the cross section.  Between drive lanes, bike lanes, parking, sidewalks, and utilities; boulevards are often squished or eliminated entirely.  A four to five foot wide planting strip is essentially the minimum width necessary for a tree to survive the day to day abuse that comes from living between a street and a sidewalk.  Much narrower and the tree roots will likely disturb curbs and sidewalks and be subject to mechanical damage from snow plows, car doors, and lawnmowers.

 

Possible Crown Spread

Estimated canopy spread relative to soil volume.  Image Courtesy of Casey Trees

Assuming a typical spacing range for boulevard trees, you are looking at about 200 – 500 ft3 per tree of oxygenated growing media for the roots, which is not enough to support most large canopy trees.  Here is a rule of thumb, taken from the excellent book Trees in the Urban Landscape: Site Assessment, Design, and Installation – you want 2 ft3 of soil for every ft2 of canopy area.  So for a tough urban tree like a honeylocust with a 30 foot wide canopy, you would want about 1,413 ft3 of soil.  Naturally, this volume isn’t absolutely necessary, but the lack of oxygenated soil is one of the main reason why the average life expectancy for an American street tree has been estimated to be between 7 and 15 years.  Also, the long and narrow boulevards with compacted urban soils lead to root systems that lack structural integrity and are less tolerant of drought and disturbance.

 

Tree Blowdown

Case in point…

This is photo from the big windstorm that blew through South Minneapolis last June, when the combination of 80 mph winds and saturated soils resulted in significant tree blowdown.  Likewise, the changing weather patterns related to global warming are resulting in longer droughts and fewer, but more intense rain storms.  This is likely to further stress out the urban forest and mean that species and practices that have worked in the past may not cut it in the future.

 

Tree Island Too Small

Undersized Tree Pit.  Image Courtesy of the UVMCollege of Natural Resources

Another factor in the short shelf life of trees is the challenging establishment period, when trees are most vulnerable to drought and disturbance.  This is particularly true for trees that are planted in highly disturbed urban sites, where compacted soil acts as a barrier to growth and limits access to water and oxygen, immediately stunting the tree and leading to decline and death.  This condition is common in undersized tree pits typically specified in parking lots or commercial areas, where grading, filling, and construction activity create soil conditions that cannot sustain long-term growth.  Compound this with shoddy nursery stock and low-bid planting methods and you have created a disposable landscape, where every year or two you pull one tree out of its coffin and put a new one back in without fixing the underlying problem.

In response to these challenges, urban foresters are developing new techniques that improve tree longevity and are enhancing the value of trees as a component of complete streets.  Many of these systems are designed to make more efficient use of underground soil volumes.   The most well-known systems are the suspended pavement or structural soil systems.  Paved surfaces require a certain level of soil compaction to remain stable, while root systems require looser soils that can cycle water, oxygen, and nutrients.  In addition to engineered systems for new tree plantings, there are also retrofit techniques that can enhance the growth of existing trees. These systems allow healthy root structures to thrive underneath streets and sidewalks.

 

z Screen Shot 2011-09-11 at 11.56.02 AM cu struct

CU Structural Soils underneath a Sidewalk.  Image Courtesy of Cornell University Horticulture Department

Structural soils describe a variety of compactable aggregate/soil mixes that maintain significant void space while remaining stable enough to support pavement.  The technique originated in Europe as a way to improve tree survival rates in dense urban areas.  When paired with cobblestones or permeable pavements, water can percolate directly to the root zone.

 

WWA_Pacific-Avenue

Back filling soil media around the structural pedestals of a Silva Cell.  Image courtesy of Deeproot.com

Suspended pavements involve the installation of reinforced pedestals that support the pavement above, with uncompacted growing media back filled into the voids.  This approach has proven to be ideal for tree growth, particularly in spots where space is at a premium and the benefits justify the substantial up-front costs.

 

FB_train-and-tree-trench

Central Corridor Stormwater Tree Plantings.  Image Courtesy of Capital Region Watershed District

Both of these systems have been successfully adapted to also function as stormwater management practices that incorporate underground storage and passive irrigation.  The significant amount of void space is the growing media can be engineered to retain and utilize large volumes of stormwater runoff.  The Tree Plantings recently installed along the Central Corridor are designed to deliver stormwater to the root zone of the trees via a network of underground pipes and then infiltrate it into the ground when possible.

 

Airspade-trench

A utility trench created with an air spade. Image Courtesy of Treelogic.au

For existing trees, there are techniques that protect the established root mass and create pathways for new roots to grow in healthy directions.  Due to the need to fix failing sidewalks and repair gas or water lines, street trees experience ongoing soil disturbance well after establishment.  Air Spades are an interesting tool that uses compressed air to blast the soil away from root structures without doing significant harm.  They minimize damage during construction and can be used to allow for retrofit soil amendments.  Another application is using them to dig trenches under sidewalks or other obstructions to allow tree roots to colonize untapped soil areas outside the right of way.

 

manchurian_alder_tree

The ‘Prairie Horizon’ Manchurian Alder, a new drought tolerant introduction from NDSU.

Another area of relentless progress is the breeding and cultivation of new tree specimens that are adapted to thrive in tough urban conditions.  At the core of this effort is the need to stay ahead of climate change.  If the climate of Minnesota is going to become comparable to that of Kansas by 2050, then there is the need to plant trees that are tough as nails and can handle drought.  The Urban Forestry Outreach and Research Nursery at the UMN is dedicated to introducing new tree varieties into wider circulation and improving arboriculture practices.  They have an excellent website filled with cutting edge research and practical guidance that is worth checking out.

 

Misc-tree-018

Bare root stock building root mass in a Community Gravel Bed.  Image Courtesy of trees.umn.edu

One example is the work they have done on Community Gravel Beds.  This is a cost-effective cultivation technique designed to improve the survival rate of new tree plantings.  By temporarily planting bare root trees in an irrigated gravel matrix, the tree rapidly develops dense networks of fine fibrous roots that dramatically increase the tree’s ability to uptake water and nutrients during its establishment period.  It can then be transplanted later in the growing season more easily and at a lower price than a containerized or ball and burlap tree.  Furthermore, the tree will likely have a superior root structure in the long term and the beds can easily be established and subsequently deconstructed on a variety of sites.

What unites all of these approaches is the effort paid towards understanding what trees need not just to survive, but to thrive.  Rather than merely selecting specimens that are hardy enough to survive in awful conditions, we are figuring out how to engineer soil and water systems that will help trees have a long and healthy life while solidifying their standing in a crowded right of way.

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2 Responses to Soil and Water in the Life of a Street Tree

  1. Walker Angell
    Walker Angell April 3, 2014 at 2:45 pm #

    Great article. Two questions on your Cornel drawing. Is the poured sidewalk permeable? If not, do they assume precip will run off the edge and still make it down to the soil underneath?

    Can the drain be buffered in some way to encourage more water to percolate down to aquifers instead of a storm sewer (which around here I believe almost always then gets flushed down the Mississippi)?

    • Samuel Geer
      Samuel Geer April 3, 2014 at 2:51 pm #

      Walker,

      Thanks for the interest. The sidewalk doesn’t need to be permeable, but it helps a lot in terms of getting water and O2 down to the roots. The underdrain is often elevated above the bottom of the system based on what the feasible infiltration rate of the soils is or what level the water is allowed to ‘bounce’ to to provide irrigation or storage retention. However, this kind of system still provides excellent attenuation of high flows so that instead of a massive rush all at once the drainage is released at a much slower rate.

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